This specification relates to facilitating content searching.
The Internet provides access to a wide variety of resources. For example, video files, audio files, web pages for particular subjects, book articles, and news articles are accessible over the Internet. A search system can identify resources in response to a search query that includes one or more search terms or phrases. The search system ranks the resources based on result scores corresponding to the resources and provides search results that link to the identified resources. The search results are typically ordered for viewing according to the ranking selected based on the result scores. The result scores can be computed, for example, based on how well data corresponding to the resource matches the search query and on measures of quality of the resources.
Image resources are resources that are provided in an image file format (e.g., JPEG (Joint Photographic Experts Group), TIFF (Tagged Image File Format), Bitmap, or other image file formats). To search image resources, a search system can determine how well images match the text of the search query, for example, using textual content of a web page in which the image is located, and sometimes based on relevance feedback associated with the image. For example, an information retrieval score measuring the relevance of a text query to the content of a web page can be combined with a click through rate of an image presented on that web page to generate an overall search result score for the image.
Textual content associated with an image can often be a reliable indicator of a topic and/or subject matter to which the image is related. However, it is possible that images unrelated to the query may be identified in search results responsive to the query if the textual content mischaracterizes the content of the image or is otherwise unrelated to the image. Therefore, images may be selected to be referenced in search results in response to search queries that are unrelated to the topic specified by the search query.